1. Field of the Invention
The present invention relates to a sheet conveying apparatus, for example, in which a cam member is used to press one roller of a pair of rollers for conveying a sheet against the other roller and separate one roller from the other-roller, and an image reading apparatus for reading an image on the sheet conveyed by the sheet conveying apparatus. Furthermore, the present invention relates to a cam member driving device for operating an operating member according to rotation of a cam member.
2. Description of Related Art
A document conveying apparatus (i.e., a sheet conveying apparatus) for feeding a document to a reading position is provided in an image reading apparatus equipped with “a running and reading function” of reading an image on a document (i.e., a sheet), which is fed to a reading glass and passes the reading position, by a reader staying at the reading position.
The image reading apparatus equipped with the running and reading function is adapted to read the document conveyed by the document conveying apparatus. Therefore, deflection occurring on the document being conveyed has caused an image deviation on an output image. This is because a shock of rushing into the roller pair disposed in the document conveying apparatus of the fore end of the document or passage of the rear end of the document causes a fluctuation in document conveying speed. In particular, since the rear end of the document is abruptly released from being pressed (i.e., held) by the roller pair when the rear end of the document passes through the roller pair, the image deviation has been liable to be generated. In the case of a thin document, the image deviation has been particularly liable to be generated.
As measures to solve the above-described problem has been known to release the holding of the document by the roller pair before the rear end of the document passes through the roller pair. In this case, since there has been a possibility of marked generation of the image deviation when a holding pressure is abruptly released, the holding pressure must be gradually reduced. Thus, it is construed that the rollers are gradually pressed with each other and gradually separated from each other by slowly rotating a cam having an inclination formed for releasing the pressure of the roller pair by a driving motor.
With the above-described configuration, the shape of the cam having the inclination need satisfy the following requirements: (1) when the roller pair starts to hold the document, the document can be securely conveyed; (2) conveying force is applied to the roller pair such that no image deviation is caused by the fluctuation in document conveying speed during reading the document; and (3) no holding force of the roller pair is exerted on the document when the document is separated from the roller pair.
Consequently, the cam must be frequently formed into such a shape as to increase a fluctuation in load to be exerted on the driving motor.
In the above-described configuration, the cam must be rotated in such a manner as to separate the roller pair while providing a timing of the document being conveyed. As a consequence, the cam need be located at a position of a predetermined rotational angle. In other words, the cam can be stopped at any rotational position, for example, immediately after a power source is turned on or after jamming of the document is cleared. In view of this, it is necessary to perform an initial operation to move the cam to the position of the predetermined rotational angle or a position at which the cam can be speedily moved to the position of the predetermined rotational angle. In conveying the document, the cam is started to be rotated from the position of the predetermined rotational angle while providing the timing of the conveyance of the document. Incidentally, during the conveyance of the document (i.e., during a normal operation), the cam performs a separating operation and a pressurizing operation of the roller pair. Therefore, the cam can be smoothly rotated by setting the driving motor in such a manner as to obtain rotating torque required for the rotation of the cam from the position of the predetermined rotational angle.
However, when the initial operation is performed to rotate the cam to the position of the predetermined rotational angle during waiting (i.e., during standing by) or the like, the cam is started to rotate in a state in which a current position is indefinite. Therefore, it is impossible to find the magnitude of the load of the rotating torque required for the rotation of the cam in a stop state. Consequently, when the driving motor is started, for example, in the state in which the cam is located at the position of the rotational angle at which the load of the rotating torque is highest, the rotating torque of the driving motor becomes insufficient, thereby causing a danger of a power swing of the driving motor. The accidental power swing of the driving motor induces an initial error, so that the document conveying apparatus is stopped accordingly. Otherwise, there has arisen a problem that the driving motor cannot be restored from the state of the power swing.
As measures for solving the above-described problems, there has been known a sheet conveying apparatus provided with a plurality of position detecting means for detecting a rotational position of a cam, in which the rotational speed of the motor is decreased in the vicinity of a position at which a maximum load of the cam is generated, thereby rotating the cam at a high rotating torque (see, for example, the prior art disclosed in FIGS. 4 and 5 of Japanese Patent Application Laid-Open (JP-A) No. 62-127270). In this case, it is necessary to provide the plurality of position detecting means for detecting the rotational position of the cam, thereby raising a problem of an increase in cost.
As other measures, there has been known a device in which the rotational speed of a motor is halved by initially starting the motor at several hundreds of pulses by 1-2 phase excitation at the time of the initial start of the motor; to which a large load is applied, by utilizing the characteristics of the motor, thereby increasing generating torque, followed by switching to 2 phase excitation (see column 8, line 11 to column 9, line 4 disclosed in JP-A No. 59-43766). In this case, there has arisen a problem of a complicated control.
That is to say, in a cam member driving device, when a driving motor is started to be rotated from a position of a rotational angle at which a load is largest in the cam having a large load fluctuation, the torque of the driving motor becomes insufficient, thereby causing a power swing of the driving motor, which is thus stopped due to an initial error. Consequently, the conventional cam member driving device has required excessive electric power at the time of starting acceleration of the motor or a complicated control. Moreover, since there are provided the plurality of position detecting means for detecting the position of the cam, a cost has been increased.
An object of the present invention is to provide a cam member driving device or a sheet conveying apparatus capable of performing an initial operation without requiring excessive electric power or a complicated control.
Another object of the present invention is to provide an image reading apparatus provided with the above-described sheet conveying apparatus in a simple structure at a reduced cost.